my 2 cents worth
You are not different than must of us shadetree mechanics. DON'T do the gears yourself. The cost and probability of failure is too high. Besides, you will probably need some new tools you don't have (but then again this is a good excuse to say "Hay noney... I need these tools to save money" sort of like having coupons)
I speak from no experience other than watching some friends that have done it. It is not difficult, but it requires experience to do it well, you need the tools and you will make additional trips to the parts store to get the right shims. If you DON'T do it right you will "feel it" in the drive and eventually in the wallet when you have it re-done.
Sorry to sound so negative about this, but this is one of those jobs I think it is wiser to get knowledgable and most important experienced people to do.
Why do you want to go to a lower gear ratio?
Do you want to do more 4 wheelin, towing, or just faster off the line?
Have you made, or plan on making the other modifications - larger tires, engine mods.
I ran my stock 3.73 gears (marked on the tag on the differential) with 32" tires for a year before going to the 4.56 gears and 33" tires. This was in my grand plan - I was prepared to put up with this trade-off knowing I would be "fixing" it in about a year.
I don't have the tables in front of me, but from my experience I suspect that you could comfortable run 31" tires with 3.73 gears, without loosing much power.
32" tires and 3.73 gears was not bad, but it did affect my shift points my RPM range, and the gears I ran around town in. It think I did a lot more shifting than I wanted, and I was often running higher RPM's than desired, especially on the highway and going up grades.
32" tires and 4.10 gears would have been very comfortable to drive and given good power to the road.
31" tires and 4.10 gears would be pretty nice, probably higher RPM's than desriable, but not out of the proper range. You would have some good pulling force.
33" tires and 4.56 gears run much better, my shift points are more normal and my RPM's are only a little higher than stock, but not bad. Grades are still a problem because the 4.0 jst does not produce the torque required running big tires.
I want to swap my stock gears on my 91 EB to 4:56. I don't know what my stockkers are? I think 3:72 or something like that? Is this swap something I can do my self? ( I know how to turn a wrench and follow directions). I will take it to a shop if it is complicated. I am all for savin' a few bucks if ya know what I mean.
